Opal glasses for light extraction

ABSTRACT

Opal glass compositions and devices incorporating opal glass compositions are described herein. The compositions solve problems associated with the use of opal glasses as light-scattering layers in electroluminescent devices, such as organic light-emitting diodes. In particular, embodiments solve the problem of high light absorption within the opal glass layer as well as the problem of an insufficiently high refractive index that results in poor light collection by the layer. Particular devices comprise light-emitting diodes incorporating light scattering layers formed of high-index opal glasses of high light scattering power that exhibit minimal light attenuation through light absorption within the matrix phases of the glasses.

This application is a continuation of U.S. patent application Ser. No.13/669,715 filed on Nov. 6, 2012, which claims the benefit of priorityunder 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/558,101filed on Nov. 10, 2011 the content of which is relied upon andincorporated herein by reference in its entirety.

BACKGROUND

Light-emitting diodes (LEDs), particularly including organic lightemitting diodes (OLEDs), are of great interest for both display andlighting applications. However, due to the high refractive index of OLEDmaterials, reduced light outputs caused by light trapping within eitherthe OLED materials or other material layers within the devices remain asignificant problem.

A number of light-scattering approaches for improving the extraction oflight from LED and OLED devices have been proposed. These include, inaddition to the use of roughened light scattering surface layers, resincovering layers containing light-scattering inclusions, bonded ceramiccovering layers of light-scattering particles, and layers formed of opalor other light-scattering glasses. Each of these proposed solutionspresents its own problems, however, including increased materials costs,limited layer durability, and/or added processing complexity.

Opal glasses typically comprise one or more light scattering phasesevenly dispersed in a fully encapsulating glass matrix. It was thoughtthat this combination of features could provide an impermeable, durableand stable light scattering material for OLED applications. However suchglasses have not found substantial commercial use for these applicationsdue to a number of actual and potential problems. Significantshortcomings of conventional opal glasses include, for example, arelatively low refractive index absent the use of expensive,index-increasing glass modifiers, a need in some cases to usesupplemental heat treatments to develop a useful level of lightscattering at low glass thicknesses, and undesirable light attenuationby the matrix glass or light-scattering phase(s) in glass layers ofhigher thickness.

SUMMARY

Although the light emission requirements for display and lightingapplications vary considerably, both would benefit from improved lightextraction from such devices. Embodiments described herein provide asolution to many of the problems connected with the use of opal glassesas light-scattering layers in electroluminescent devices such as LEDsand OLEDs. In particular, embodiments solve the problem of high lightabsorption within the opal glass layer as well as the problem of aninsufficiently high refractive index that results in poor lightcollection by the layer. Particular embodiments comprise light-emittingdiodes incorporating light scattering layers formed of high-index opalglasses of high light scattering power that exhibit minimal lightattenuation through light absorption within the matrix phases of theglasses.

Some embodiments comprise an electroluminescent device such as a lightemitting diode comprising a light-scattering layer for enhancing theextraction of light from the device, wherein the light-scattering layercomprises an opal glass having a refractive index greater than 1.6.Further, the properties of the opal glass may comprise a matrixattenuation coefficient below 1 cm⁻¹ at a light wavelength of 400 nm,e.g., an attenuation coefficient below 0.4 cm⁻¹, or even below 0.04cm⁻¹, in some embodiments.

Further embodiments comprise an OLED or LED device comprising adjoiningelectron transport and hole transport layers, a transparent electrode incontact with at least one of the transport layers, and alight-scattering layer in contact with the transparent electrode,wherein the light-scattering layer comprises an opal glass ofLi₂O—TiO₂—SiO₂ composition or Li₂O—TiO₂—SiO₂—P₂O₅ composition, the glasscomprising a vitreous or amorphous TiO₂ light-scattering phase, having arefractive index greater than 1.6 and having an attenuation coefficientnot exceeding 0.04 cm⁻¹ at a light wavelength of 400 nm.

Opal glasses comprising a Li₂O—TiO₂—SiO₂ or Li₂O—TiO₂—SiO₂—P₂O₅composition offer unique advantages as light-scattering materials forimproving light extraction from OLED devices. In some embodiments, theTiO₂ constituent of these glasses performs a dual function, acting toincrease the refractive index of the glass as well as to provide anefficient light-scattering TiO₂ phase as the glass is cooled from themelt. The TiO₂-containing phases in these glasses, which may comprise avitreous or crystalline structure depending on the thermal history ofthe glass, can in many cases be formed without supplemental heattreatments, and are sufficiently dense that a high degree of lightscattering can be secured even at very low glass thicknesses.

Further embodiments comprise opal glass sheet of Li₂O—TiO₂—SiO₂ orLi₂O—TiO₂—SiO₂—P₂O₅ composition. In some embodiments, the glass sheetcomprises Li₂O, TiO₂, SiO₂, and, optionally, P₂O₅. In some embodiments,the glass sheet consists predominantly of Li₂O, TiO₂, SiO₂, and,optionally, P₂O₅. By consisting predominantly is meant that thedisclosed oxides make up at least about 65, 70, 75, or 80% by weight ofthe glass forming the sheet. The opal glass sheet comprises a vitreousor crystalline TiO₂ light scattering phase and has a thickness notexceeding about 500 μm, 400, 300, 200, 100, or in some embodiments notexceeding 50 μm, a refractive index of at least 1.6, 1.7, 1.8, 1.9, or2.0, and an attenuation coefficient below about 0.01, 0.02, 0.03, 0.04,0.05, 0.06, 0.07 or 0.08 cm⁻¹ at a light wavelength of 400 nm. Suchsheets can serve as device-supporting substrates or as light-scatteringcovering layers for OLED devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are further described below with reference to the appendeddrawings, wherein

FIG. 1 is a schematic illustration of a design for top-emitting OLEDdevice;

FIG. 2 is a schematic illustration of a design for a bottom-emittingOLED device;

FIG. 3 is a schematic illustration of a second design for abottom-emitting OLED device;

FIG. 4 is a graph plotting light transmission through selectedlight-scattering opal glasses;

FIGS. 5A and 5B comprise a photomicrograph and corresponding lightintensity data for an electroluminescent sample with no opal glass coversheet (left hand side) and with an embodied opal glass cover sheet(right hand side);

FIGS. 6A and 6B comprise a photomicrograph and corresponding lightintensity data for an electroluminescent sample with no opal glass coversheet (left hand side) and with an embodied opal glass cover sheet(right hand side); and

FIG. 7 present light scattering data for a selected light-scatteringopal glass.

DETAILED DESCRIPTION

The problem of light trapping in OLEDs is well known. In the so-calledtop-emission device design wherein the light is emitted upwardly from adevice disposed on a light-reflecting substrate, a very large percentageof the generated light can be trapped in upper device layers, includingOLED or electrode layers, which are typically of higher refractive indexthan protective covering layers or the surrounding atmosphere. Ideally,a device covering layer of equal or higher refractive index than, forexample, the top electrode layer can promote the efficient collection oflight emitted through the electrode, but some means for efficientlyextracting light from the device covering layer is still required.

In bottom-emission device designs the bottom OLED layer is typically incontact with a transparent bottom electrode disposed on a transparentsubstrate composed, for example, of glass. The bottom electrode isgenerally of relatively high refractive index in comparison to the glasssubstrate, however, so trapping of generated light within OLED layers orthe electrode can again occur. In accordance with one study, a bottomemission diode design evaluated using a classical ray model calculatedthat 51% of the initially generated light is trapped within the activeorganic and electrode layers, 31.5% of the light is trapped within theglass substrate, and only 17.5% of the light is coupled out into theair. Thus, in both bottom- and top-emission device designs, some meansfor reducing internal reflections and allowing more of the generatedlight to be emitted from the diode structures is needed.

The benefits of more efficient light extraction from OLEDs and otherelectroluminescent devices go beyond increasing light emission at agiven operating power level. The service lives of such devices areinfluenced by device drive voltages. Therefore, improving the lightextraction efficiency of such devices would allow operation at lowerpower levels while achieving the same light outputs. A properlyengineered light scattering system, whether in the form of a surfacescattering layer or a bulk scattering material, can significantlyimprove light extraction from these devices.

While the opal glass light-scattering layers disclosed in accordancewith the present description can be employed to improve the function ofa wide variety of light-emitting electroluminescent devices, they offerparticular advantages when incorporated into LED and OLED devices.Accordingly, the following descriptions include specific illustrativeembodiments of OLED designs even though the benefits are not limitedthereto.

The opal glass materials employed in accordance with the presentdisclosure typically comprise at least two phases, with the major phasein particular embodiments comprising a silicate glass phase ofLi₂O—TiO₂—SiO₂ composition. In some embodiments, a Li₂O—TiO₂-SiO₂ glassphase comprises a phase wherein the glass comprises Li₂O, TiO₂, andSiO₂, i.e., with those oxides making up at least about 65, 70, 75, or80% by weight of the glass. The light-scattering phase of TiO₂ generallycomprises only a minor volume fraction (i.e., less than 10% by volume)of the material.

Most opal glasses within the above ranges of composition have meltingtemperatures not exceeding 1400° C., making them well suited formanufacture by conventional melting and forming methods. Thus they canbe conveniently formed into glass sheets within a thickness range, forexample, of about 50-500 μm, or about 50, 100, 200, 300, 400, or 500 nmthick. Such thicknesses provide a level of light scattering that issufficient to significantly improve light extraction from current OLEDdevices even where relatively rapid quenching of the glass duringsheet-forming is employed. Moreover, controlling the thermal history ofthe glass permits tuning of the opacity during forming, securingscattering through the thicknesses of the glass layers as appropriatefor any particular device application.

Securing a light attenuation below about 0.01, 0.02, 0.03, 0.04, 0.05,0.06, 0.07 or 0.08 cm⁻¹ at a light wavelength of 400 nm in these glassesrequires that the concentration of light-attenuating species in theglass, particularly including light absorbing transition metals such asiron, be controlled. For that reason opal glass sheet herein, as well asthe light-scattering opal glass layers in the OLED devices, willgenerally have compositions comprising less than about 0.5, 0.4, 0.3,0.2, 0.1, or 0.05% total weight of oxides of iron, nickel, chromium, andmanganese. The use of low-iron silica sources such as low-iron sand canhelp to achieve the necessary limitations on transition metal oxidelevels.

Particular embodiments of opal glass compositions for glass sheets anddevice layers provided in accordance with the present descriptioncomprise those wherein the opal glasses have compositions comprising, inweight percent, about 60-85 SiO₂, 10-30 TiO₂, and 5-15 Li₂O. Inaddition, however, the glasses may further comprise conventional glassmodifiers in limited proportions to the extent not adversely affectingthe light-scattering efficiencies of the glasses. As specific examples,these glasses may further comprise, in weight percent, up to about 10%total of oxides selected from the group consisting of B₂O₃, Al₂O₃, Na₂OK₂O, and/or CaO, and up to about 5% total of oxides selected from thegroup consisting of MgO and Al₂O₃. The opal glass compositions mayfurther comprise conventional glass fining aides such as As₂O₃ and/orSb₂O₃.

Depending upon the particular device designs selected for the inclusionof light-scattering layers as herein disclosed, the use of opal glasseshaving refractive indices even higher than about 1.6, 1.7, 1.8, 1.9, or2.0, may be advantageous. Opal scattering layers intended for use inOLED devices incorporating transparent electrodes of very highrefractive index should be of higher refractive index. For theseapplications it can be useful to modify the compositions of thedisclosed Li₂O—TiO₂—SiO₂ glasses by adding refractive-index-increasingconstituents to the glass. Particular embodiments of such compositionsinclude those wherein the opal glass further contains at least onerefractive-index-modifying constituent selected from the groupconsisting of La₂O₃, Nb₂O₅, Ta₂O₅, PbO and Bi₂O₃. Opal glass sheets anddevice layers having refractive indices of at least 1.7, or in someembodiments at least 1.8, are examples of higher index scattering layersof tuned refractive index that can readily be provided through the useof such constituents.

As noted above, light-scattering opal glass sheets or layers provided inaccordance with the present description can be advantageously employedin a wide variety of OLED device designs. These include use in directcontact with top-emission and bottom-emission OLED devices, or ascovering and/or encapsulating layers for the electrodes used in suchdevices. Light-scattering opal glass sheets can also providedevice-supporting substrates, for example, in bottom-emission OLEDdevice configurations.

Illustrative examples of OLED device designs incorporatinglight-scattering opal glass light extraction layers or substrates inaccordance with the disclosure are schematically illustrated in FIGS.1-3 of the drawings. FIG. 1 presents a schematic cross-sectional view,not in true proportion or to scale, of a top-emitting OLED device 10,the predominant direction of light emission from the device beingindicated by the upwardly directed arrow in the figure.

Device 10 comprises an electron transport layer 12 disposed on and inelectrical contact with a hole transport layer 14 to form alight-emitting junction. Layer 12 is covered by and in electricalcontact with transparent electrode 16 while layer 14 is in electricalcontact with opposite light-reflecting electrode 18, those electrodesoperating to power the device. All of the layers are supported on adevice substrate 20.

In accordance with the present disclosure, device 10 is further providedwith a light-scattering opal glass covering layer 22, that layer beingdisposed over and in contact with transparent electrode 16. The opalglass layer operates to collect and then scatter light traversing thetransparent electrode when the device is activated, thereby increasingthe amount of light extracted from the device.

FIG. 2 of the drawings presents a similar schematic cross-sectional viewof a bottom-emitting OLED device 30, the predominant direction of lightemission from the device being indicated by the downwardly directedarrow in the figure. Device 30 again incorporates an electron transportlayer 12 in contact with a hole transport layer 14 to provide alight-emitting junction. In the case of device 30, however, lightreflecting electrode 18 is disposed over electron transport layer 12while transparent electrode 16 is disposed beneath hole transport layer14 to allow for the collection and downward emission of light generatedwithin the device.

Also included in device 30 in accordance with the present disclosure isa light-scattering opal glass layer 22, disposed between transparentelectrode 18 and a transparent device substrate 20. Light-scatteringlayer 22 operates to extract and downwardly scatter light emitted fromthe diode junction that collects within transparent electrode 18. Thusthe amount of light directed into and downwardly traversing transparentsubstrate 20 is increased by layer 22.

FIG. 3 of the drawings presents a schematic cross-sectional view of analternative bottom-emitting OLED device 50, the predominant direction oflight emission from the device again being indicated by the downwardlydirected arrow in the drawing. The arrangement of electron and holetransport layers 12 and 14 as well as the positioning of transparentelectrode 20 and light-reflecting electrode 18 in device 50 areanalogous to the arrangements and positioning employed in the design ofdevice 30. However, device substrate 22 a in device 50 provides a dualfunction in the OLED design, providing both a substrate for supportingthe device and a light-scattering functionality for the purpose ofincreasing the amount of light emitted by the device. For those purposessubstrate 22 a comprises a thin sheet of a Li₂O—TiO₂—SiO₂ opal glassincorporating a light-scattering TiO2 phase as disclosed above, thatsheet acting to extract light from transparent electrode 20 and scatterit downwardly from the device.

Illustrative examples of glass compositions suitable for providinglight-scattering opal glass layers or substrates within the scope of thepresent description are set forth in Table 1 below. The glasscompositions are reported in parts by weight on the oxide basis ascalculated from the batches for melting the glasses.

TABLE 1 Opal Glass Compositions Sample ID Oxide Component Glass A (% wt)Glass B (% wt) SiO₂ 59.84 31.47 TiO₂ 24.93 4.66 Li₂O 9.97 0 B₂O₃ 4.99 0Sb₂O₃ 0.5 0 Al₂O₃ 0 11.11 MgO 0 8.25 ZnO 0 0.56 P₂O₅ 0 43.86

In a typical glass manufacturing procedure, batches for the aboveglasses are compounded, ball-milled, and melted in platinum crucibles at1400° C. The melts are then cast into glass sheets and annealed at about490° C. to provide stress-free glass castings.

The light-transmitting characteristics of the glasses thus provided areevaluated by conducting light transmission measurements on opal glasssheets of known thickness. Measurements are taken on both glass samplesas initially cast, and on similar samples subjected to supplemental heattreatments to maximize the development of light-scattering TiO₂ phasesin the glasses. Due to the rapid development of light-scattering phasesin these glasses, even the samples as cast can exhibit a relatively highdegree of opacity due to light scattering. As noted above, a wide rangeof scattering levels can be produced in these glasses, both throughvariations in composition and variations in the thermal history of theglass as affected by variations in the forming methods and heattreatments used in manufacture.

FIG. 4 of the drawings illustrates the range of variations in lighttransmission that can be exhibited by the glasses set forth in Table 1.FIG. 4 is a graph presenting curves plotting the light transmissions ofsamples of these glasses over the wavelength range of visible light asreported on the horizontal axis of the graph. The measured levels oflight transmission through the samples are reported in percenttransmission on the vertical axis of the graph.

Transmission Curve A in FIG. 4 plots light transmission through a 1.2 mmthick sample of Glass A from Table 1 above as cast from the melt, whiletransmission Curves A′ and A″ reflect light transmission through samplesof the same glasses, at thicknesses of 0.17 mm and 0.175 mm,respectively, after four-hour heat treatments at 600° C. and 650° C.,respectively. Curve B plots light transmission values as measured on a0.125 mm thick sample of Glass B from Table 1 as cast from the melt. Theextensive development of light-scattering TiO2 phases indicated by thetransmission measurements on these glasses confirm that even very thinopal glass layers or substrates are all that will be required to achievehigh levels of light scattering and light extraction from OLEDs andother electroluminescent devices incorporating such glasses.

The effectiveness of light-scattering Li₂O—TiO₂—SiO₂ opal glass layersfor enhancing the extraction of light from electroluminescent devices isshown by the emitted light intensity data presented in FIGS. 5a and 5bof the drawings. Each of those figures comprises a photomicrographresulting from a three-second exposure to a photoluminescent sampleduring activation by a UV light source. Each figure further comprises aplot of the light extraction data as measured from the photomicrograph.

Preparation of the photoluminescent samples for measurement comprisesproviding each sample with a coating of index-matching oil and thencovering the right-hand segments of each sample with a light-scatteringopal glass sheet of the Glass A composition reported in Table 1 above.The opal glass sheet employed in FIG. 5A is of 100 μm thickness and thesheet in FIG. 6A is of 200 μm thickness.

The relative light extraction efficiencies of the two samples areindicated by the enhanced brightness of the opal-glass-coveredright-hand segments of the illuminated fields shown in thephotomicrographs (FIGS. 5B and 6B). The data plotted in the graphs showa brightness enhancement of 2.1× for the 100 μm opal glass sample ofFIG. 5A, and an enhancement of 1.4× for the 200 um opal glass sample ofFIG. 6A. The marked enhancements in light extraction provided by both ofthe light-scattering opal glass sheets are evident from these figures.

Yet a further advantage imparted to OLED devices provided in accordancewith particular embodiments is that the opal glass light-scatteringlayers comprising the TiO₂ light-scattering phases are essentially freeof specular transmission and exhibit substantiallywavelength-independent Lambertian scattering. FIG. 7 of the drawingsplots the value of the bidirectional transmittance distribution function(ccBTDF) for transmitted light scattered over scattering angles from−90° to +90° through a 0.1 mm-thick sample of an opal glass having thecomposition of Glass A from Table 1 above. Uniform (Lambertian)scattering is shown at each of the plotted blue (400 nm), green (600nm), orange (800) and red (1000) light wavelengths, with all plots beingabsent intensity peaks at 0° which would be indicative of speculartransmission through the sample.

While the disclosure has referenced particular embodiments of OLEDdevices and light-scattering opal glass layers and substrates, it willbe apparent from those descriptions that the embodiments presented aremerely illustrative of the various devices and glasses that may beselected by those of skill in the art for the practice of embodimentswithin the scope of the appended claims.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “metal” includes examples having two or moresuch “metals” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are embodiments of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. Thus, if a class of substituents A,B, and C are disclosed as well as a class of substituents D, E, and F,and an example of a combination embodiment, A-D is disclosed, then eachis individually and collectively contemplated. Thus, in this example,each of the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F arespecifically contemplated and should be considered disclosed fromdisclosure of A, B, and C; D, E, and F; and the example combination A-D.Likewise, any subset or combination of these is also specificallycontemplated and disclosed. Thus, for example, the sub-group of A-E,B-F, and C-E are specifically contemplated and should be considereddisclosed from disclosure of A, B, and C; D, E, and F; and the examplecombination A-D. This concept applies to all aspects of this disclosureincluding, but not limited to any components of the compositions andsteps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific embodiment or combination of embodiments of the disclosedmethods, and that each such combination is specifically contemplated andshould be considered disclosed

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present disclosurewithout departing from its spirit and scope. Since modificationscombinations, sub-combinations and variations of the disclosedembodiments incorporating the spirit and substance of the disclosure mayoccur to persons skilled in the art, the disclosure should be construedto include everything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. An electroluminescent device comprising alight-scattering layer for enhancing the extraction of light from thedevice, wherein the light-scattering layer comprises an opal glasshaving a refractive index greater than 1.6 and a matrix attenuationcoefficient below 0.04 cm⁻¹ at a light wavelength of 400 nm.
 2. An OLEDdevice comprising adjoining electron transport and hole transportlayers, a transparent electrode in contact with at least one of thetransport layers, and a light-scattering layer in contact with thetransparent electrode, wherein the light-scattering layer comprises anopal glass of Li₂O—TiO₂—SiO₂ composition or Li₂O—TiO₂—SiO₂—P₂O₅composition comprising a vitreous or amorphous light-scattering TiO₂phase, the glass having a refractive index greater than 1.6 and anattenuation coefficient not exceeding 0.04 cm⁻¹ at a light wavelength of400 nm.
 3. An OLED device in accordance with claim 2, wherein the opalglass has a composition comprising less than 0.1% total weight of oxidesof iron, nickel, chromium, and manganese.
 4. An OLED device inaccordance with claim 2 wherein the opal glass has a refractive index ofat least 1.8.
 5. An OLED device in accordance with claim 2 wherein theopal glass has a composition comprising, in weight percent, about 60-85SiO₂, 10-30 TiO₂, and 5-15 Li₂O.
 6. An OLED device in accordance withclaim 5 wherein the opal glass further comprises, in weight percent, upto about 10% total of oxides selected from the group consisting of B₂O₃,Al₂O₃, Na₂O K₂O, and/or CaO, and up to about 5% total of oxides selectedfrom the group consisting of MgO and Al₂O₃.
 7. An OLED device inaccordance with claim 5 wherein the opal glass further comprises atleast one refractive-index-modifying constituent selected from the groupconsisting of La₂O₃, Nb₂O₅, Ta₂O₅, PbO and Bi₂O₃.
 8. An OLED device inaccordance with claim 2 wherein the opal glass light-scattering layercomprising the TiO₂ light-scattering phase is free of speculartransmission and exhibits substantially wavelength-independentLambertian scattering.
 9. An opal glass sheet of Li₂O—TiO₂—SiO₂ orLi₂O—TiO₂—SiO₂—P₂O₅ composition comprising a vitreous or crystallineTiO₂ light scattering phase and having a thickness not exceeding 500 μm,a refractive index of at least 1.6, and attenuation coefficient below0.04 cm⁻¹ at a light wavelength of 400 nm.
 10. An opal glass sheet inaccordance with claim 9 having a composition containing, in weightpercent, less than 0.1% total of oxides of transition metals selectedfrom the group consisting of iron, nickel, chromium and mangnese.
 11. Anopal glass sheet in accordance with claim 9 having a thickness notexceeding 500 μm and having a refractive index of at least 1.8.
 12. Anopal glass sheet in accordance with claim 9 having a compositioncomprising, in weight percent, about 60-85 SiO₂, 10-30 TiO₂, and 5-15Li₂O.
 13. An opal glass sheet in accordance with claim 12 furthercomprising, in weight percent, up to about 10% total of oxides selectedfrom the group consisting of B₂O₃, Al₂O₃, Na₂O K₂O, and/or CaO, and upto about 5% total of oxides selected from the group consisting of MgOand Al₂O₃.
 14. An opal glass sheet in accordance with claim 12 whereinthe glass phase contains at least one refractive-index-modifyingconstituent selected from the group consisting of La₂O₃, Nb₂O₅, Ta₂O₅,PbO and Bi₂O₃.
 15. An opal glass sheet in accordance with claim 12having a melting temperature not exceeding 1400° C.
 16. An opal glasssheet in accordance with claim 12 having a thickness in the range of50-100 μm.